Production of hollow fibers by co-electrospinning of cellulose acetate

The study concerns the use of the electrospinning technique for the formation of
cellulose acetate hollow nanofibers. These hollow fibers are used to manufacture
hollow fiber membranes. Important properties that should be inherent to these
hollow-nanofibers include excellent permeability and separation characteristics, and
long useful life. They have potential applications in filtration, reverse osmosis, and
the separation of liquids and gases.
It is apparent from the available literature on electrospinning and co-electrospinning
that the diameter and the morphology of the resulting fibers are significantly
influenced by variations in the system and process parameters, which include the
solution concentration, solvent volatility, solution viscosity, surface tension and the
conductivity of the spinning solution.
The materials used include cellulose acetate (CA) (concentration = 11~14 wt %),
(feed rate = 1~3 ml/h), acetone:dioxane (2:1) and mineral oil (feed rate = 0.5~1
ml/h) with core and shell linear velocity of 2 and 0.7 mm/min respectively. These
materials were used as received without further purification.
The co-electrospinning setup used comprised a compound spinneret, consisting of
two concentric small-diameter capillary tubes/needles, one located inside another
(core-shell/co-axial design). The internal and external diameters of the inside and
outside needles were 0.3 and 1.2 mm respectively (0.3 mm shell/core gap space).
The liquids CA (shell) and mineral oil (core) are pumped to the coaxial needle by a
syringe pump, forming a compound droplet at the tip of the needle. A high voltage
source is used to apply a potential of several kilovolts over the electrospinning
distance. One electrode is placed into the spinning solution and the other oppositely
charged (or neutral) electrode attached to a conductive collector. If the charge build
up reaches approximately 15 kV the charged compound droplet, (poorly conductive
polymer solution) deforms into a conical structure called a Taylor cone. On further
increasing, the charge at the Taylor cone to some critical value (unique to each
polymer system) the surface tension of the compound Taylor cone is broken and a core-shell jet of polymer solution ejects from the apex of the Taylor cone. This jet is
linear over a small distance, and then deviates in a course of violent whipping from
bending instabilities brought about by repulsive charges existing along the jet length.
The core-shell jet is stretched and solvent is evaporated and expelled, resulting in the
thinning and alignment of the fiber. Ultimately dry (most solvent having been
removed) submicron fibers are collected in alignment form in a simple collector
design (water bath).
The shell to core solution flow rate ratio was chosen according to the parameter
response of shell-core diameter of the resulting fibers in order to achieve an optimal
hollow structure after removal of the mineral oil core. The mineral oil of the dry
collected core-shell fibers is removed by immersion in octane. The aforementioned
response is determined by measurement of core-shell diameters using scanning
electron microscopy (SEM) and transmission electron microscopy (TEM).
The obtained results showed that the ability of the spinning solution to be
electrospun was directly dependent on its concentration and the feed rate of the
spinning solution and also parameters such as the spinning distance and type of
solvents used. The preferable polymer solution concentration is 14 wt %, shell feed
rate of 3 ml/hr, core feed rate of 0.5 ml/hr (2 and 0.7 mm/s core and shell linear
velocity respectively), applied voltage of 15 KV, spinning distance of 8 cm and
coaxial spinnerets having internal diameters of 0.3 mm and 1.2 mm core and shell
needles respectively (0.3 mm shell/core gap space) have been found to make
uniform cellulose acetate hollow fibers with an average inside and outside diameter
of approximately 495 and 1266 nm, respectively.